Method and apparatus to generate a clock-based transmission

ABSTRACT

A method and apparatus are disclosed to generate and/or receive ultra-wide bandwidth (UWB) pulses using a digital clock.

BACKGROUND OF THE INVENTION

Ultra Wideband (UWB) systems transmit signals across a much widerfrequency range than conventional systems. The bandwidth of the UWBsignal may be equal to at least 20% of the center frequency or more than500 MHz. UWB transmission operates by transmitting and receivingextremely short duration bursts of radio frequency (RF) energy. Eachpulse is extremely short in duration, e.g., 0.1 to 4 nanoseconds (ns).

Devices known in the art for generating UWB transmission use diode-basedpulse generators, typically including a step-recovery diode or amicrowave-tunnel diode, which are triggered by a clock. In suchconfigurations, although the diodes may have good switching orpulse-generation characteristics and can be tuned to cover a widebandwidth, they are difficult to integrate, particularly incomplementary metal oxide semiconductor (CMOS) devices.

Other UWB transmission devices known in the art, which may be used inhigh-power UWB radar transmitters, are based on pulsed-power technology,e.g., capacitive discharge circuits, transformer switches andtransmission line switches, and/or light-activated semiconductorswitches. These devices are generally expensive and difficult tointegrate with CMOS. In addition, the high power limits for UWB radartransmission make this type of device unsuitable for use in commercialcommunication systems, for example, because of limits set by the FederalCommunications Commission (FCC).

Another type of transmitter known in the art is based on a gatedoscillator. In this approach, in which a local oscillator is gated, thecenter frequency of the spectrum can be controlled by the oscillatorfrequency; however, the gated oscillator approach is not flexible. Forexample, the center frequency cannot be digitally derived, thegeneration of multiple tones requires multiple phase-locked loops (PLL),and the PLL implementation may consume a significant amount of power, asit cannot be turned off between pulses to maintain loop stability.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanied drawings in which:

FIG. 1 is a simplified block diagram of a transmitter using a gatedclock pulse generation circuit as a basis to modulate data in a UWBcommunications system according to exemplary embodiments of the presentinvention;

FIG. 2A is a schematic diagram illustrating unmasked bipolar modulatedpulses according to exemplary embodiments of the invention;

FIG. 2B is a schematic diagram of mask including low duty-cycle bipolarmodulated pulses transmitter according to exemplary embodiments of thepresent invention;

FIG. 2C is a schematic diagram of bipolar modulated pulses passingthrough a bandpass filter according to exemplary embodiments of thepresent invention;

FIG. 3 is a schematic block diagram of a gated clock pulse generationcircuit to modulate data in a UWB communications system according toexemplary embodiments of the present invention;

FIG. 4 is a schematic block diagram of a direct-sequence spread spectrum(DSSS) UWB transmitter implementation according to exemplary embodimentsof the present invention;

FIG. 5 is a schematic block diagram of a DSSS UWB receiverimplementation according to exemplary embodiments of the presentinvention;

FIG. 6A is a schematic time domain representation of a pulsed orthogonalfrequency-division multiplexing (OFDM) waveform based on simultaneoustransmission of multiple carriers, according to exemplary embodiments ofthe present invention;

FIG. 6B is a schematic time domain representation of a pulsed orthogonalfrequency and time division multiplexed (OF/TDM) waveform based ontransmission of non-overlapping, or partially overlapping, pulsedwaveforms, according to exemplary embodiments of the present invention;

FIG. 6C is a schematic frequency domain representation of a pulsed OFDMor OF/TDM ultra-wideband waveform including a concatenation of multiplesub-bands, according to exemplary embodiments of the present invention;

FIG. 7 is a schematic time domain representation of an OF/TDMtransmitted waveform, including interleaved sub-band pulses, eachoccupying a different frequency band, according to exemplary embodimentsof the present invention;

FIG. 8A schematically illustrates a time-domain representation of aHanning-shaped sub-band waveform using an embodiment of an OF/TDM systemin accordance with exemplary embodiments of the present invention; and

FIG. 8B is a schematic frequency-domain representation of a shapedsub-band OF/TDM waveform in accordance with exemplary embodiments thepresent invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn accuratelyor to scale. For example, the dimensions of some of the elements may beexaggerated relative to other elements for clarity or several physicalcomponents included in one functional block or element. Further, whereconsidered appropriate, reference numerals may be repeated among thefigures to indicate corresponding or analogous elements. Moreover, someof the blocks depicted in the figures may be combined into a singlefunction.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the present invention may be practiced without these specificdetails. In other instances, well-known methods, procedures, componentsand circuits may not have been described in detail so as not to obscurethe present invention.

Embodiments of the invention provide a method and apparatus forgenerating ultra-wide bandwidth (UWB) pulses using a digital clock inconjunction, for example, with a UWB transmitter and/or receiver. Forexample, in some embodiments of the present invention, theimplementation of transmitters and transmission methods may be based ondigital processing. These implementations may be integrated intoexisting devices using complementary metal oxide semiconductor (CMOS)processes. Moreover, although the scope of the present invention is notlimited in this respect, some embodiments of the present invention maybe configured to operate within the boundaries set by the FederalCommunications Commission (FCC) or any other regulatory body. Inaddition, embodiments of the present invention may use a digitallyderived center transmission frequency.

The invention is described below in the context of a wireless personalarea networking (WPAN) communication system. However, it will beapparent to persons skilled in the art that the invention may also besuitable for any other system or device that can drive a digital clock.

An embodiment of the present invention includes an implementation of anUWB transmitter, wherein a digital clock is used as a basis fortransmission. The use of a digital clock, as opposed to the prior artuse of analog elements, may significantly simplify complementary metaloxide semiconductor (CMOS) integration of the transmitter circuit, forexample, because the digital clock may be generated in a straightforwardmanner using a digital phase locked loop (PLL). Moreover, the samecircuit may be used at the receiver end of the communication system withsubstantially the same reduction in complexity and ease of integrationenabled by the transmitter of some embodiments of the invention. Forexample, in a correlator-based receiver, which may be the receiver ofchoice in the case of an additive white Gaussian noise environment, theclock based generator according to embodiments of the invention may formpart of the circuit performing “template-matching” to the originalpulse, reducing receiver costs.

The block diagram of FIG. 1 provides a general description of how agated clock pulse generation circuit may be used to modulate data in aUWB communications system according to embodiments of the presentinvention. A clock signal may be generated by a clock generation circuit10. Modulator 30 may combine clock signal produced by clock generationcircuit 10 with data 20. The resulting waveform may be gated and shapedby a gating and shaping unit 40, and filtered through a bandpass filter50, ultimately resulting in a transmission signal 60 having a desiredpulsed waveform. It should be noted that, in alternate embodiments ofthe invention, at least some of the blocks shown in FIG. 1 may becombined into a single circuit, for example, functions of clockgeneration circuit 10 and modulator circuit 30 may be integrated into asingle circuit, or any other integration and/or disintegration offunctions of the different blocks shown in FIG. 1 may be performed, inaccordance with specific design considerations.

In one embodiment of the invention, the digital clock signal may begenerated by a digital phase lock loop (PLL), as is known in the art.The clock signal may then be modulated, in which case a rational numberof periods of the clock signal may form the signal waveform. Forexample, as shown in FIG. 2A, for bipolar modulation, the waveform mayinclude a certain pattern to represent ‘1’ in the data and an invertedpattern to represent ‘0’.

The shape of the signal waveform may be controlled by gating the clockwith a periodic switch, so that a rational number of periods of theclock may form a pulse. The gating of the clock may be controlled usingsimple digital logic, including AND and OR circuits, making the gatingmechanism efficient to operate at high speeds and easy to integrate intoCMOS. This may lower the duty cycle of the signal waveform and mayenable some or all the advantages of a pulse-based UWB system, includinga lower fading margin and lower inter-symbol interference. A possibleincrease in the peak-to-average power ratio (PAR) of the signal, due tosuch gating, may be controlled by using a sufficiently high duty cycle.In addition, the gating may control the spectral bandwidth of thesignal, because the width of the gate varies roughly inversely with thespectral bandwidth. FIG. 2B shows bipolar modulated pulses with a lowduty cycle.

In some exemplary embodiments of the invention, the shape of the signalwaveform may be controlled by a shaping function to obtain a smoothersignal envelope. Appropriate shaping of the signal may help reducehigher-order harmonics in the signal spectrum caused by the transmissionof the digital clock. In some embodiments of the present invention, thedigital clock may be followed by an analog band-pass filter in order toreduce higher-order harmonics and to maintain the signal within thelimits of a given spectral mask, e.g., as required by Part 15 rules forunlicensed devices developed by the Federal Communications Commission(FCC). FIG. 2C shows low duty-cycle bipolar modulated pulses afterapplying a bandpass filter, in accordance with embodiments of theinvention.

There are generally at least three design parameters that may bemodified to generate a UWB waveform of particular characteristics: clockfrequency (f_(c)); gating period (T_(g)); and shaping function. Inembodiments of the present invention, all three characteristics may becontrolled and/or varied by controlling and/or varying various clockfrequencies which are used to generate the UWB waveform, as describedbelow.

The fundamental clock frequency may determine the center frequency ofthe modulated waveform. In some embodiments of the present invention,the clock frequency may be a variable so that the center frequency ofthe modulated waveform may be altered as necessary. This provides theflexibility of easily moving the UWB waveform to a different frequencyband, which may be important if UWB frequency bands differ in differenttransmission areas, e.g., different countries. This may also provide theflexibility of moving to unoccupied frequency bands in a particularlocation to minimize interference.

The gating period may determine the bandwidth of the occupied UWBwaveform (e.g., bandwidth ˜1/Tg). In some embodiments of the presentinvention, the gating period may be a variable in order to providefurther flexibility of the transmission. Having a gating period that isa multiple of the clock period enables use of the same clock to generateboth the fundamental clock and gating clock; however, a gating periodthat is a rational multiple of the clock period may also be easilyderived. Also, changing the gating period and center frequency providesa convenient mechanism for changing the occupied frequencies of the UWBwaveform, allowing the system to adaptively change to accommodatedifferent interference environments.

The shaping function, which may be implemented in conjunction with thegating function, or as a separate circuit, may provide the flexibilityfor spectrum shaping by smoothing the envelope of the transmitted pulse.For example, in some embodiments of the present invention, a windowedfunction may be used to smooth the time domain pulse and limithigh-frequency components in the waveform. The windowed function may forexample be a Hanning window or a single cycle of a sinusoid. This mayreduce the side-lobe energy of the transmitted waveform.

In some embodiments of the present invention, a windowed function may bebased on a filtered version of a gating function, which may be derivedfrom the clock signal, as described above. This function may be usefulfor a sub-band implementation of the present invention where multipleUWB pulses occupy adjacent frequency channels, as shown in FIGS. 6A, 6B,6C, 8A, and 8B.

Embodiments of the present invention may be used in several types of UWBsystems. One example is the pulse-based systems discussed above.Embodiments of the present invention suitable for use in such a systemmay involve an implementation of the clock generation scheme depicted inFIG. 3. A PLL 100 generates a high-frequency clock signal 110, which maybe modulated with data 120 into a series of bipolar modulated pulses. Inthe embodiment shown in FIG. 3, either the pulse or the inverted pulseis chosen, depending on the data, e.g., ‘0’ or ‘1’, using digitalinverter and buffer elements 130 and 140, respectively, and AND and NANDgate elements 150 and 160, respectively. The resulting waveform is thengated and shaped. A gating and shaping unit 190 may include alow-frequency version of a low frequency clock 180 which may gate, viaclock converter 170, a predetermined multiple of the period of the highfrequency clock 110. For example, the high-frequency clock 110 may runat 7 GHz, while the low-frequency clock 180 may run at 3.5 GHz. In thisexample, every two periods of the high-frequency clock forms a singlepulse. The gating and shaping unit 190 may establish the bandwidth ofthe emitted UWB signal by controlling the rise time of the pulses. Thegated pulses may be passed through an optional bandpass filter 200, forexample, if needed to meet out of band emission requirements.

Other embodiments of the present invention may be implemented inconjunction with a direct-sequence spread spectrum (DSSS) UWB system, asdepicted in FIGS. 4 and 5. This embodiment may use high-speed digitalclocks to generate chip sequences to form direct sequence informationsymbols at rates on the order of, for example, 3 to 5 GHz. In someembodiments, code-word length and chip rates may be varied ortransmitted in parallel to produce information transfer rates of 500megabits per second (Mbps) or higher. Codes may be produced, forexample, by associating a ‘0’ with a certain sequence of encoded pulsesand a ‘1’ with a different sequence of encoded pulses. For example, eachbit may be represented by a series of 15 pulses, wherein the phase ofeach pulse may be either inverted or not inverted. Different bits may berepresented by a different sequence of 15 pulses, referred to herein asa code. Different users may also use different sets of codes, forexample, to allow multiple users to transmit on the same frequencysimultaneously without causing significant interference to each other.

FIG. 4 shows an embodiment of the present invention that may yield adata transmission rate of, for example, 313 Mbps. In the exemplaryembodiment shown, data to be transmitted 400 is input into a 1-bit shiftregister 402. Although the scope of the invention is in no way limitedin this regard, a 16-bit code word, e.g., “code word 0” (block 406) or“code word 1” (block 408) may be selected by a word selector 404depending on the data symbol output by the shift register 402. Theselected code word may be received by a multiplexer (Mux) 412, forexample, a sixteen-to-one multiplexer (shift register) 412 operated by a5 GHz clock 410, thereby resulting in a 5 GHz processing rate oftransmitter 414. Thus, if 16 bits are transmitted at a frequency of 5GHz, the information transmission rate is 313 Mbps. Those skilled in theart will recognize that other transmission rates are possible and thatthe center frequency of the RF emission may be thus controlled.

FIG. 5 depicts an embodiment of a DSSS UWB receiver. Antenna 500, whichmay be a dipole antenna or another type of antenna, may receive the UWBsignal and may amplify it via amplifier 502. Multiplier or input mixer504 multiplies a signal incoming via amplifier 502 with a templatesignal, which may be generated by a code generator 512 and passedthrough a bandpass filter 508. The output signal of the multiplier 504may be filtered through a lowpass filter 506 to remove higher orderharmonics. When time synchronized, this signal may recover the originaltransmitted information. A decimator or divider 514 may optionally beused to clock an analog-to-digital converter (ADC) 510 at the symbolrate rather than at the chip rate, which may be substantially higher.Variable digital delay circuitry 516 may be used to search and match thetiming of the receiver to that of the transmitter. The DAC 518 maycontrol the amount of delay. Code generator 512 and bandpass filter 508may be used to form a template generator for correlation with theincoming signal. A clock source, for example, having frequency 5 GHz maybe used to drive code generator 512. The code generator 512 may producea template signal that may be correlated with a received waveform atmultiplier or input mixer 504. Variable digital delay circuitry 516 maybe used to search for and match the timing of the received signal, afterwhich the signal may be digitized by ADC 510, e.g., for DSSS processingsuch as, for-example, channel equalization and symbol detection, as isknown in the art. The digital signal output from the ADC may be providedto a digital signal processor 520 for processing.

Yet another embodiment of the present invention may be implemented inconjunction with a sub-band UWB system (SB-UWB), also known asOrthogonal Frequency and Time Division Multiplexed (OF/TDM) or pulsedorthogonal frequency-division multiplexing (OFDM). FIGS. 6A, 6B, 6C, 7,8A and 8B show examples of sub-band waveforms that may be used inconjunction with embodiments of the invention. In these embodiments, thespectrum is divided into a number of sub-bands which, for example, mayoccupy a bandwidth on the order of 500 MHz. By concatenating several,for example, six or eight, sub-band waveforms, an ultra-widebandwaveform occupying, e.g., approximately 4 GHz of spectrum, may begenerated. The sub-band waveforms may, for example, be transmittedsimultaneously, or they may, for example, be interleaved in time andtransmitted in non-overlapping time intervals, or they may, for example,be transmitted during partially overlapping intervals. Some or all, ornone, of the sub-band waveforms may have the same bandwidth. The centerfrequency of the sub-band waveforms and the bandwidth of the waveformsmay be controlled, for example, using a gated clock circuit, e.g., asdescribed above.

The schematic diagrams of FIGS. 6A, 6B and 6C show time and/or frequencydomain representations of an exemplary sub-band implementation ofembodiments of the invention. FIG. 6A shows a time domain representationof a pulsed OFDM waveform based on simultaneous transmission of multiplecarriers, that may be implemented using gated clock circuits accordingto exemplary embodiments of the present invention. In the example shownin FIG. 6A, a concatenated OFDM waveform s(t) is transmitted during aportion of the pulse period, depending on the duty cycle.

FIG. 6B shows a time domain representation of a pulsed OF/TDM waveform,i.e., sub-banded or multi-banded waveform, based on the transmission ofnon-overlapping, or partially overlapping, pulsed waveforms, whichwaveform may be implemented using gated clock circuits according toexemplary embodiments of the present invention. In the example shown inFIG. 6B, the six sub-band waveforms s1(t) to s6(t) are transmittedduring alternating portions of the pulse period, substantially fillingthe period. In the example shown, each sub-band waveform fills afraction of the pulse period, e.g., one sixth in FIG. 6B, depending onthe number of sub-band waveforms transmitted.

FIG. 6C shows a frequency domain representation of a pulsed OFDM orOF/TDM waveform on the order of several GHz including a concatenation ofmultiple sub-bands, that may be implemented using gated clock circuitsaccording to exemplary embodiments of the present invention. In theexample shown, six sub-bands may be transmitted at center frequencies500 MHz apart, where some or all have bandwidth 500 MHz.

In another example, depicted in FIG. 7, six SB-UWB waveforms may begenerated using clock frequencies varying between 3.5 and 6 GHz, whichmay be gated with a period of 2 nanoseconds (ns), resulting inbandwidths on the order of approximately 500 MHz. FIG. 7 shows thesesub-band waveforms transmitted during non-overlapping periods. Suchnon-overlapping transmission may have the advantages of reducing thepeak-to-average power ratio (PAR) of the signal and enabling alternateimplementations, e.g., implementations where sub-bands may be generatedsequentially, e.g., only one at a time.

In the embodiment shown in FIG. 7, sub-bands may be modulated bydifferent data bits, resulting in a potentially very high throughput.Also, the coverage of sub-bands may be changed within the UWB frequencydomain by changing the clock frequency used to generate the sub-bandwaveform. For example, if such a SB-UWB system is implemented inconjunction with an IEEE 802.11a wireless local-area-network (WLAN)device that operates in the 5.15-5.35 GHz frequency band, a sub-bandoverlaying the 5.15-5.35 GHz band may be adjusted to a higher frequencyby changing the clock frequency. (See Institute of Electrical andElectronics Engineers (IEEE) Standard 802.11a-1999, “Wireless LAN MediumAccess Control (MAC) and Physical Layer (PHY) Specification: High-speedPhysical Layer in the 5 GHz Band”). FIGS. 7 and 8A show an exemplarytime domain representation of the transmitted waveforms, where thesub-band pulses are interleaved in time.

In these examples, the UWB sub-bands may be generated using digitalclocks, as described previously, with the output filtered through, forexample, a 3.0-6.5 GHz Bessel bandpass filter (FIG. 7) and shaped usinga Hanning window (FIG. 8A). FIG. 8A shows an example of the time-domainrepresentation of a shaped sub-band waveform having a Hanning shape andusing an embodiment of an OF/TDM system in accordance with embodimentsof the present invention. FIG. 8B shows a frequency-domainrepresentation of a shaped sub-band OF/TDM waveform in accordance withembodiments of the present invention. Those of ordinary skill in the artwill recognize that there are various methods to generate the multipleclock frequencies, as may be used in this embodiment of the invention,including multiple analog PLLs, digital PLLs, or multiple phases of asingle PLL, or any other suitable method.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.Embodiments of the present invention may include other apparatuses forperforming the operations herein. Such apparatuses may integrate theelements discussed, or may comprise alternative components to carry outthe same purpose. It will be appreciated by persons skilled in the artthat the appended claims are intended to cover all such modificationsand changes as fall within the true spirit of the invention.

1. A method comprising: accepting a digitally produced clock signal anddata; and deriving a direct sequence ultrawide bandwidth signal from thedigitally produced clock signal and data, wherein deriving the ultrawidebandwidth signal comprises: digitally producing the clock signal;modulating the clock signal with the data to obtain a modulated signal,wherein modulating the clock signal with data comprises inverting theclock signal to obtain an inverted clock signal and modulating theinverted clock signal with the data using an AND gate; and shaping themodulated signal to obtain the ultrawide bandwidth signal.
 2. The methodof claim 1, wherein deriving the ultrawide bandwidth signal comprisesgating the modulated signal to obtain the ultrawide bandwidth signal. 3.The method as in claim 1, wherein shaping the modulated signal comprisesfiltering the modulated signal.
 4. The method as in claim 1, whereinshaping the modulated signal comprises windowing the clock signal with afunction of the modulated signal.
 5. A method comprising: receiving adigitally produced clock signal and data; deriving a direct sequenceultrawide bandwidth signal from the digitally produced clock signal anddata, wherein deriving the ultrawide bandwidth signal comprises:digitally producing the clock signal; modulating the clock signal withthe data to obtain a modulated signal, wherein modulating the clocksignal with data comprises buffering the clock signal to obtain abuffered clock signal and modulating the buffered clock signal with thedata using a NAND gate; and shaping modulated signal to obtain theultrawide bandwidth signal.
 6. The method of claim 5, wherein derivingthe ultrawide bandwidth signal comprises gating the modulated signal toobtain the ultrawide bandwidth signal.
 7. The method of claim 5, whereinshaping the modulated signal comprises filtering the modulated signal.8. The method of claim 5, wherein shaping the modulated signal compriseswindowing the clock signal with a function of the modulated signal. 9.An apparatus comprising: a digital clock to produce a digital clocksignal; a modulator to modulate the clock signal with data to produce amodulated signal, wherein modulating the clock signal with datacomprises inverting the clock signal to obtain an inverted clock signaland modulating the inverted clock signal with the data using an ANDgate; and a shaping unit to shape the modulated signal, wherein theapparatus is adapted for deriving a direct sequence ultrawide bandwidthsignal from the digitally produced clock signal and the data.
 10. Anapparatus as in claim 9 further comprising a bandpass filter to reducehigher-order harmonics in the modulated signal.
 11. A system comprising:a digital clock to produce a digital clock signal; a modulator tomodulate the clock signal with data, thereby to produce a modulatedsignal, wherein modulating the clock signal with data comprisesinverting the clock signal to obtain an inverted clock signal andmodulating the inverted clock signal with the data using an AND gate; ashaping unit to shape the modulated signal; and a dipole antenna totransmit the modulated signal, wherein the system is adapted forderiving a direct sequence ultrawide bandwidth signal from the digitallyproduced clock signal and the data.
 12. The system of claim 11 furthercomprising a bandpass filter to reduce higher-order harmonics in themodulated signal.
 13. The system of claim 12, wherein the bandpassfilter is a Bessel bandpass filter.